Magnetic ink has been used since long in the field of banknote printing, to confer the printed currency an additional, covert, security element. Features printed with magnetic ink lend themselves as well for machine authentication, given that magnetism can be easily sensed by electronic means.
Examples for the use of magnetic currency features are disclosed in U.S. Pat. No. 3,599,153 and in U.S. Pat. No. 3,618,765. The magnetic currency features are preferably applied through the “copperplate intaglio” printing process, which is capable of depositing a sufficiently high amount of magnetic material on the paper so as to allow for its detection and sensing.
Banknote printing is characterized by the use of the “copperplate intaglio” printing process (engraved steel die rotary printing process), which constitutes an element of security on it own and provides the printed document with a distinct tactility.
In engraved steel die rotary printing, a rotating engraved cylinder carrying the pattern or image to be printed, is supplied with ink by one or more template inking cylinders, by which a pattern of inks of different color is transferred to the printing cylinder. Subsequent to inking, any excess of ink on the plain surface of the printing cylinder is wiped off by a rotating wiping cylinder covered by a “plastisol”.
The remaining ink in the engraving of the printing cylinder is transferred under pressure onto the substrate to be printed, which may be paper or plastic material in sheet or web form. During the printing process high pressure is applied between the engraved cylinder and substrate to be printed, causing deforming (embossing) of the latter. This high pressure printing process results in the characteristic touch of a banknote.
Because of the unique characteristics of rotary intaglio printing process and of the corresponding printing machines, the inks used in this printing process must be specifically formulated.
Intaglio inks are characterized by their pasty consistency; generally the viscosity of intaglio inks for the engraved steel die printing process is comprised within 1 to 15 Pa·s, more specifically within 3 to 8 Pa·s at 40° C. at a shear rate of 1000 sec−1. Intaglio inks are also characterized by their high solids contents, of typically more than 50% by weight.
A particular problem encountered with magnetic intaglio inks is their generally rather dark appearance and thus their limited accessible color gamut, due to the dark color of the known magnetic pigments: Fe2O3 which has a brown-red color; Fe3O4 which is black; or ferrite materials which are mostly dark grey; Lighter shade pigments such as soft magnetic metallic iron also appear grey in an ink. The dark color of the known magnetic pigments does not allow for the formulation of inks having light colors, such as orange, yellow or white, and limits thus the freedom for realizing artistic designs using magnetic inks. Clear magnetic pigments, and intaglio inks comprising such pigments, are therefore highly desirable, because they would allow for magnetic printed designs in any desired shade. The available magnetic intaglio inks do not currently allow a flexible integration into colored banknote designs, as to color, surface coverage and location.
EP 1 650 042 A1 discloses an intaglio printing ink comprising magnetic flake pigment, carrying on each side a color-generating interference layer sequence. The inks of EP 1 650 042 have the advantage that inks of vivid colors can be realized, through the use of magnetic particles having such vivid interference color coatings. The pigment particles of EP 1 650 042 are, however, not resistant to corrosion, due to the exposure, at the flake edges, of the flake's metal layers to the ink medium.
A printing ink for the printing of documents by the method of engraved steel die printing, comprises, further to the pigment, other print forming ink solids containing oleoresinous components; at least one volatile organic solvent to be evaporated during or after printing; and a film forming macromolecular hydrophilic surface active composition in full or partial replacement of said oleoresinous components, in combination or not with lower molecular weight surfactants. The amount of said volatile organic solvents is less than about 15% by weight of the total weight of the printing ink. The macromolecular hydrophilic surface active composition is preferably selected from anionic substances being the salts of carboxylic, phosphonic or sulfonic acid groups in said macromolecules with metals or amines.
Copperplate intaglio inks typically comprise a significant amount of macromolecular or low molecular weight surfactant, embodied as a partially neutralized carboxylic resin, in order to i) promote the adhesion of the ink to the cellulosic printing substrate, and ii) to allow for an easy cleaning of the wiping cylinder, using a basic aqueous surfactant solution. These surfactants are usually species carrying acid functions, partially neutralized with organic or inorganic bases, with a remaining acid number. The macromolecular hydrophilic surface-active composition is preferably selected from anionic substances being the salts of partially neutralized carboxylic, phosphonic or sulfonic acid groups in said macromolecules with metals or amines. Intaglio printing inks for this printing process have been disclosed in EP 0 340 163 B1 and EP 0 432 093 B1. Alternatively intaglio inks may also contain entities carrying non-neutralized acidic groups which are only neutralized when put in contact with the basic wiping solution, such as to allow for the cleaning of the wiping cylinder.
On the other hand, these acidic or partially neutralized intaglio ink components are a frequent cause for the corrosion of pigments, in particular metallic pigments such as bronze powders. Soft magnetic carbonyl iron powder, e.g, is known not to be stable in a carboxylic resin containing intaglio ink. The partially neutralized carboxylic resin in particular supplies, on the one hand, protons for the oxidation of the iron metal, according to the equation:2-COOH+Fe→2-COO−+Fe2+H2,On the other hand it also acts as a complexing/solubilizing agent for the liberated Fe2+ ion in the ink medium, preventing it from forming an oxide protecting layer on the iron metal:Fe2++n-COO−→[Fe(—COO)n](n-2)—
This solubilization of the oxidized iron continuously liberates the metal surface for further attack. The dissolved metal, on the other hand increases the viscosity of the ink until reaching a point where the latter is no longer printable. Intaglio inks comprising metallic iron pigment tend therefore to have a decreased shelf life. A magnetic iron containing intaglio ink with higher shelf life would be highly appreciated by the printer.
There is thus a need for magnetic intaglio printing inks not showing the shortcomings of the prior art. Accordingly, it is an object of the present invention to respond to this need.